Passive ship motion stabilization system with active assist for high amplitude motions

ABSTRACT

A floating vessel has primary stabilization tanks located adjacent both sides of the vessel both fore and aft in the vessel. The primary tanks extend vertically above and below the load waterline. Each primary tank has a series of vertically spaced openings to the exterior of the vessel, the openings being both above and below the load waterline. A secondary stabilization tank is provided in association with each primary tank and extends vertically in the vessel above and below the load waterline. Each set of primary and secondary tanks is interconnected at their lower ends via a passage which has an effective water flow rate which is greater than the effective water flow rate by gravity through the openings from the primary tank. Air flow means are connected to the upper extent of the secondary tanks and are operable alternately to generate superatmospheric air pressure and subatmospheric air pressure in the secondary tanks.

United States Patent Anders June 3, I975 I PASSIVE SHIP MOTION STABILIZATION SYSTEM WITH ACTIYE ASSIST FOR HIGH AMPLITUDE MOTIONS 175! Edward ()len Anders. Houston. Tex.

lmentor Assignee Global Marine Inc.. Ins Angeles,

Calif.

[22 Filed: Feb. 28. I974 [2!] Appl. No; 446,700

[51) US. CL. II4/l25 [5 I] Int. Cl 863i] 43/06 [58] Field ot'Search ll4/l2l. I22. 125.6!

[56] References Cited L'NITED STATES PATENTS L88Xfifi? ll[l Hurt ll4/ll5 3.360.08 l/l9h8 Hilliard ll4/l25 3. 97.440 RfiJT-l Pungalila ll4/l'l5 Primun E.mrnim'i''l ryg\e M. Blix .Ivs'is'mn! lfiumi'm'r Stuart M. Goldstein Artur/m .Jgwu. or firm-Christie. Parker 8-: Hale [57] ABSTRACT A floating vessel has primary stabilization tanks located adjacent both sides of the vessel both fore and aft in the vessel. The primary tanks extend vertically above and below the load waterline. Each primary tank has a series of vertically spaced openings to the exterior of the vessel, the openings being both above and below the load waterline. A secondary stabilization tank is provided in association with each primary tank and extends vertically in the vessel above and below the load waterline. Each set of primary and secondary tanks is interconnected at their lower ends via a passage which has an effective water flow rate which is greater than the effective water flow rate by gravity through the openings from the primary tank. Air flow means are connected to the upper extent of the secondary tanks and are operable alternately to generate superatmospheric air pressure and suhatmospheric air pressure in the secondary tanks.

8 Claims, 5 Drawing Figures SHEET 2 PASSIVE SHIP MOTION STABILIZATION SYSTEM WITH ACTIVE ASSIST FOR HIGH AMPLITUDE MOTIONS FIELD OF THE INVENTION BACKGROUND OF THE INVENTION Copending application Ser. No. 400,269 filed Sept. 24, I973, is assigned to the same record assignee as the present invention and describes a passive tunable motion stabilization system which is effective to counteract wave induced pitch, heave and roll motions of a vessel. The present invention is an improvement over the motion stabilization system described in this copending application and provides an active assist to the passive system to more effectively compensate for and counteract high amplitude motions of the vessel.

Numerous arrangements have heretofore been proposed for stabilizing floating vessels. These known arrangements are of both passive and dynamic types in which ballast is caused to shift in the vessel in a manner that couteracts or damp motions induced in the vessel by wave action and the like. Most known ballast transfer stabilization systems require the transfer of mass either entirely across the beam of the vessel or back and forth along its length for counteracting roll and pitch motions, respectively. Commonly, these ballast transfer systems are tuned by the fixed structure of the system to be most effective at a selected roll or pitch frequency which normally is defined at a frequency less than the resonant pitch or roll frequency of the vessel. The passive tunable motion stabilization system described in the above identified copending application has the advantage over other similar systems in that it does not require the transfer of ballast within the vessel from one point to another; because it is tunable it has a greater operative roll and pitching motion frequency range over which it is effective to stabilize motions of the vessel.

It now appears that even where the vessel is equipped with a tunable passive motion stabilization system according to the above mentioned copending application, the system may not be effective to damp or counteract substantial motions of the vessel under conditions where the motion inducing inputs to the vessel, by wave action and the like, is cyclically in substantial resonance with the natural periods of pitch, roll or heave of the vessel. While long period wave trains, tending to resonance with the natural periods of motion of the vessel, are not commonly encountered, the motions of the vessel during such conditions of resonance are disproportionately greater than the motions experienced by the vessel when the period of the passing wave trains is substantially below the corresponding natural period of the vessel. It is more desirable to provide a motion stabilization system which is responsive to the motion inducing wave train spectrum most commonly seen by the vessel; economic considerations applicable to vessel design and operation significantly dictate this compromise. In many classes of vessels, however, especially in vessels used as floating drilling vessels, it is highly desirable that the vessel be equipped with means for stabilizing both commonly encountered wave induced motions as well as those less commonly induced motions which correspond to resonance between the period of passing wave trains and the natural periods of the vessel. A need therefore exists for a motion stabilization system which, under normal operating curcumstances. is passive in nature but which, during conditions of resonance between the vessel's natural motion periods and those of passing wave trains, is actively augmented and assisted for effective motion stabilization of the vessel during all conditions of use.

SUMMARY OF THE INVENTION This invention provides an effective, efficient and reliable motion stabilization system which satisfies the need described above. The active-assist aspects of the present stabilization system are pneumatic in nature and are so arranged as to take advantage of the monentum of water movements occurring in the passive phase of the system. Where necessary, the system can also be operated to statically adjust the pitch or heel of the vessel.

Generally speaking, this invention provides a motion stabilization system for a floating vessel which has a buoyant hull. The system includes a first tank which extends vertically in the hull from a first selected location below the hull load waterline to a second selected location above the waterline. The first tank has at least one vertical boundary thereof defined by the hull shell. A plurality of vertically spaced openings are formed through the shell from the first tank, some of these openings being located below the waterline and others being located above the waterline. A second tank extends vertically in the hull from about the elevation of the first location to about the elevation of the second location. Means are provided between the first and second tanks for water flow communication between the lower and upper tanks; these means have an effective water flow rate therethrough which is greater than the effective water flow rate through the openings from the first tank. Air flow means communicate with the upper extent of the second tank and are operable to alternately generate superatmospheric and subatmospheric pressures in the second tank.

DESCRIPTION OF THE DRAWINGS The above mentioned and other features of this invention are more fully set forth in the following detailed description of a presently perferred embodiment of the invention, which description is presented with reference to the accompanying drawings wherein;

FIG. I is a side elevation view of a vessel incorporating the present invention;

FIG. 2 is a front view of the vessel;

FIG. 3 is a detailed view of the mechanism for opening and closing the openings from the first tank to the exterior of the hull;

FIG. 4 is a sectional view taken line 44 of FIG. 3; and

FIG. 5 is a simplified cross-sectional elevation view through a set of first and second tanks of the stabilization system of this invention, and shows the connection and general arrangement of the air flow means for the second tank.

Referring to FIGS. I and 2, there is shown a vessel 10 designed for use as a drilling platform; this vessel is more fully described in copending applications Ser. No.

substantially along 399,806 filed Sept. 24, I973 and Ser. No. 400.269 filed Sept. 24. I973. both of which applications are assigned to the record assignee of the present application. While the present invention is described with particular reference to the vessel disclosed in the above-identified copending applications, this invention is not limited to use in any particular type of vessel and has application to both conventional ship-shape vessels and also that class of floating drilling vessel known as the semisubmersible."

Vessel includes a pair of vertical hulls, indicated generally at 12 and 14, joined at the top by a bridging platform indicated generally at 16. The platform includes a main deck 18 and a lower deck 20 which are rigidly tied together by box girder construction. The vertical hulls l2 and 14 are secured at the upper ends thereof to the lower deck 20 of the platform 16. The assembly of the vertical hulls l2 and 14 and the bridging platform 16 is structurally reinforced and joined together in a unitary rigid structure by means of suitable tubular reinforcing members, including a plurality of horizontal cross members 22 extending between the hulls l2 and 14 above the waterline. The horizontal cross members 22 in turn are tied to the underside of the platform 16 by upright tubular sections 24 and diagonal cross braces 26.

The vertical hulls I2 and 14 terminate at their lower ends in torpedo-shaped sections 28 and 30, respectively. Each of the torpedo-shaped sections includes a cylindrical portion extending the length of the hull which is of somewhat larger diameter than the beam width of the remainder of the hull. The torpedo-shaped sections have a hemispherical bow 32 at one end and a tapered stern section 34 at the other end. A propeller 36 extends beyond the end of the stern section and is surrounded by a protective shroud 38. The propellers are driven by suitable motors (not shown) mounted in the interior of the stern section. The torpedo-shaped sections at the lower ends of the hulls are rigidly joined together by horizontal cross members 40, which, as shown in FIG. 1, are preferably oval shaped in crosssection to reduce resistance to movement through the water.

Considering the design of vertical hulls in more detail, each hull is constructed of bulkheads and outer plates welded together to form a rigid watertight structure. Each hull comprises a pair of substantially parallel side walls 50 and 52 which extend vertically from beneath the lower deck 20 down to the junction with the outer skin of the cylindrical lower section 28. The bow 53 and stem 55 of the hulls are semi-cylindrical in form, for example, to reduce drag in moving through the water.

The interior of each hull is compartmentalized by a series of vertical bulkheads S7. The vertical bulkheads are used in combination with the side walls 50 and 52 of each hull to form vertically extending tanks 58, 60, 62 and 64. The tanks are arranged in pairs with two of the tanks in the forward part of the hull and two of the tanks in the aft portion of the hull. The tanks are sealed off at either end by bottom walls 66 (FIGS. 1 and 5) and top walls 68 (FIGS. 3 and 5). The top and bottom walls are positioned such that the tanks extend vertically substantially above and below the waterline.

Each of the vertical tanks is provided with a series of vertically spaced openings 70 located on the outboard side of the respective hulls I2 and 14. As best seen in FIGS. 3 and 4. the size of the openings may be varied by rotatably supported plates or louvers 72 positioned inside the tanks opposite each of the openings 70. Each plate 72 is supported on a shaft 74 extending horizontally across the center of each of the openings 70. the shaft 74 being supported at either end from the inside of the side plates forming the outer side wall of the hull by suitable angle brackets 76 and 78 (FIG. 4). Journaled on the shaft 74 are a pair of supporting arms 80 and 82 which are secured at one end to the plate 72 and are connected at the other end by a shaft 84. A vertically extending connecting rod 86, journaled on each of the shafts 84, extends upwardly through an opening 88 in the top wall 68 of the tank. An hydraulic actuating cylinder 90 is pivotally supported at one end in a clevis 91 mounted on the top wall 68. The piston rod 93 of the cylinder 90 in turn is pivotally coupled to the top of the connecting rod 86, as indicated at 92. The top of the connecting rod 86 is also movably supported from the side wall 52 by a link 94.

As shown in FIGS. 3 and 4, the movable plates 72 are shown in the closed position in which the openings are substantially closed off. Energizing of the hydraulic cylinder 90 lifts the connecting rod 86, thereby rotating the arms and 82 so as to move the plates 72 into an open position, as indicated by the dotted lines in FIG. 3.

The tanks 58, 60, 62, and 64, and the fore and aft portions of each of the hulls operate to reduce the vertical movement of the vessel. This vertical movement of heave response is due to the cyclical change of buoyancy in the vessel resulting from the rise and fall of the water by passing wave action. For a conventional vessel there is no control over such changes in buoyancy as the boat rises and falls. However, with the tanks of the present invention open to the sea, water flows into and out of the tanks as the water level rises and falls, so that the total weight of the vessel also varies in a cyclical manner. As the change in buoyancy causes the level of the vessel in relation to the level of the water to change, water either flows into the tanks or flows out of the tanks through the openings 70. The change in water level in the tanks and therefore the change in the weight of the vessel varies cyclically at the same frequency as the cyclical changes of buoyancy due to wave action. By adjusting the openings to the tanks, the phasing of the two actions can be adjusted so that an increase in buoyancy is counteracted by an increase in weight. The apparent change of buoyancy therefore stays more nearly constant, thereby reducing the net force causing the level of the ship to rise and fall with wave action.

An additional damping effect is achieved by the partially opened Iouvered plates in the openings. The plates when opened extend at an angle to the side of the vessel, providing a scooping action below the level of the water, generating an energy absorbing vortex when the water tries to move past the edges of the louver plates and the edges of the openings. This action of the louvered plates acts as effective damping for roll and pitch of the vessel.

In the preferred embodiment, all of the openings 70 through the sides 52 are shown as being equal in area. However, the openings need not be of equal size. The top openings, for example, generally function only to allow air to enter and escape from the volume above the surface of the water and may therefore be made smaller in area without materially changing the operation. Also, the size of the openings below the waterline may be adjusted in size independently of the openings above the waterline.

The foregoing description of vessel corresponds to the description appearing in copending application Ser. No. 400,269. The foregoing description, therefore, is a description of the passive aspects of the present actively-assisted passive stabilization system. The passive aspects of the present stabilization system function essen tially independently of the active aspects of the stabilization system during most conditions of use of vessel 10.

In the foregoing description, each of tanks 58, 60, 62 and 64 is a primary or first stabilization tank. Each primary stabilization tank is provided in conjunction with a secondary active stabilization tank according to this invention, such as tank 95 shown in FIG. 5. It will be understoood that the following description of the set of primary tank 58 and secondary tank 95 will suffice as a description for the several sets of primary and secondary tanks provided in vessel 10.

Secondary tank 95 is provided in vertical hull 12 adjacent to primary tank 58 and extends vertically in hull 12 above and below the load waterline 96 of the vessel. Preferably, tank 95 has top and bottom walls which are disposed at the same elevation in hull 12 as the top and bottom walls, respectively, of the adjacent primary stabilization tank. Preferably tanks 58 and 95 have a common vertical wall 97 between them. The lower extents of tanks 58 and 95 are in water flow communication, as by an opening 98, which may conveniently be defined by terminating common wall 97 above the bottom wall of the two adjacent tanks. Opening 98 is sized sufficiently that the water flow rate between the two tanks, particularly during operation of the air flow means 99 (described in greater detail below) for tank 95, is greater than the effective water flow rate by gravity through the several openings 70 provided from primary tank 58 to the exterior of bull 12. Valve means, such as a plurality of louver-like valve plates I00, are provided in association with opening 98 for isolating secondary tank 95 from primary tank 58, as desired. During periods when primary tank 58 is isolated from secondary tank 95, the secondary tank may be pumped dry by use of suitable pumps and ducts (not shown) coupled to the lower extent of the secondary tank. Preferably, secondary tank 95, like primary tank 58, has vertical walls so that the horizontal area of the secondary tank is substantially uniform at all locations along its height. It will be apparent from an examination of FIG. 5 that when valve means 100 are open to provide communication between the lower extents of the primary and secondary tanks, the two tanks resemble a U-tube such that air pressure changes in the secondary tank have corresponding effects in primary tank 58. The volumetric capacity of secondary tank 95 may be as great as that of primary tank 58 although the secondary tank may be made smaller in capacity ifdesired.

Air flow means 99 is located in hull 12 above the top wall of secondary tank 95. The air flow means includes an air blower I01, illustrated schematically in FIG. 5, which is driven by a suitable power source such as a diesel engine or electric motor. Blower I01 has a suction connection 102 and a discharge connection 103. A common air supply and air exhaust duct 104 communicates with an upper extent of the secondary tank and is coupled to the suction and exhaust connections I02 and 103 of blower I00 by suitable ducting 105 and by valves 106 and 107, respectively. Ducting 105 also provides coupling of the blower suction and discharge con nections separately to atmosphere. as through lower deck 20, via valves 108 and 109, respectively. As indicated by broken lines in FIG. 5, valves 106, 107, I08, and 109 are operated by a control mechanism 110. The nature of the control provided at any given time by control mechanism 110 over valves 106-109 is defined by the operative state of a mode selection device 111 which is operatively coupled to the control mechanism. Preferably, a single mode selection device 111 is provided in vessel 10, but each pair of primary and secondary stabilization tanks has its own air flow means and each air flow means has its own control mechanism 110, the single mode selection device 111 being connected to the several air flow control mechanisms.

Under applicable rules concerning the construction and stability ratings of floating vessels, it is desirable that primary tank 58 be defined to have a relatively small horizontal area so as to keep the stability deduction due to the free surface effect of water in the primary tank to an acceptable value, this is particularly true where the vessel in which the stabilization system is used is a conventional ship-shape vessel rather than the twin hulled vessel 10 shown in the drawings. This stability oriented criteria applicable to primary tank 58 is but one of several factors facing the naval architect. The effect of these factors is to strongly suggest or require that primary tank 58 be sized somewhat smaller than the dimensions ofa passive stabilization tank sufficient, according to the operative principles described above, to counteract induced vessel motions in those situations where the period ofwave trains inducing motions of vessel 10 correspond to the natural periods of motion of the vessel. Thus, as a practical matter, primary tank 58 will usually be sized so that it is less effective to counteract vessel motions during times when resonance exists between the vessel motion periods and wave train periods than it is when the period of passing wave trains is less than the natural period of the vessel in heave, pitch or roll, for example. For these reasons, in those relatively few (perhaps about five percent) times when the period of passing wave trains is in resonance to the natural motion periods of the vessel, the differential hydrostatic head between the interior and exterior are of primary tank 58 per so may not exist adequately to cause sufficient gravity flow of water into and out of the primary tanks through openings 70. In these instances, the primary tank is less effective to counteract and stabilize induced motions of the vessel than when shorter period waves are encountered.

Secondary tank and its air flow means cooperate with primary tank 58, when valve means are open, to actively augment the differential head existing between the primary stabilization tank and the exterior of the vessel during resonance conditions. In this manner the overall motion stabilization system may have as great or greater effectiveness at resonance conditions as during nonresonant conditions when shorter period wave trains are encountered by the vessel. The effect of the combination of primary and secondary tanks 58 and 95 is to provide a motion stabilizing and counteracting effect on the vessel which cooresponds to the presence of a substantially larger passive stabilization tank, but without the penalty of an increased stability deduction which would be associated with a larger passive tank. The deduction from stability associated with the free surface effect in a tank within a vessel. such as passive stabilization tank 58, increases with the cube of the dimensions of the tank in the direction of the mt tion of interest.

During those times when the increased motion stabilizing effect of secondary tank 95 is desired, air blower 101 is activated and valves 107 and 108, on the one hand. and 106 and 109, on the other hand, are oper ated in tandem by control mechanism 100. As the water level outside the primary tank rises relative to load waterline 96, as by the movement of a long period wave (having a high troughto-crest distance) past the vessel, valves 106 and 109 are open whereas valves 108 and 107 are closed. This operation of the valves of air flow means 99 couples the suction connection of blower 101 to the secondary tank and couples the discharge connection of the blower to atmosphere so that subatmospheric pressure is generated in the upper extent of secondary tank 95. (In this regard, the height of the wave crest above the waterline of the vessel is represented by broken line 113 in FIG. 5, whereas the elevation of the trough of the same wave below load waterline 96 is indicated by broken line 114.) As subat mospheric pressure is generated in secondary stabilization tank, water rises in the secondary tank to elevation 115 which lies a distance A 11 above the load waterline of the vessel, a distance greater than wave crest 113.

After the wave crest passes, the effective hydrostatic head in the pair of tanks will be greater than the effective hydrostatic head provided by merely adding the volume of secondary tank 95 to that of primary tank 58. Thus. the motion inhibiting effect of the water held captive in the primary and secondary tanks will be greater than that of the water in the primary tank alone.

Conversely, after the trough 113 of the wave has passed the vessel, the operative states of air flow regulating valves 106-109 is reversed so that the discharge connection of blower 101 is coupled to secondary tank 105 so that superatmospheric pressure is generated in the tank. At this time, the level of water in the secondary tank is driven to a lower level 116 which lies a greater distance A 11 below the load waterline than wave trough 114. Thus, the motion inhibiting effect of the pair of tanks relative the next approaching wave crest is greater than that provided by primary tank 58 alone.

The effective area of opening 98 between the primary and secondary stabilization tanks is such that the effective water flow rate through the opening is greater than the gravity induced water flow rate through the several openings 70 from tank 58 to the exterior of the vessel. Accordingly, when the secondary tank is connected to the suction of blower 101, water is transferred by the subatmospheric pressure in the secondary tank from the primary tank to the secondary tank at a rate greater than the rate at which water flows into the primary tank, thereby increasing the effectiveness of the primary tank. The momentum of this water flow into the secondary tank is in the desired direction tending to reduce the required horse power of the blower drive mechanism. The greater height of level 115 is reflected as stored potential energy in the system in expectation of the arrival of a wave through at the vessel. Conversely, when the discharge connection of the blower is coupled to the secondary tank via valve 107,

water is transferred from the secondary tank to the pri' mary tank at a rate greater than that in which water flows from the primary tank to the exterior of the vessel. thereby also enhancing the motion stabilization effect of the primary tank itself.

Preferably air blower 101 is operative to produce a subatmospheric pressure in secondary tank of approximately 2 pounds per square inch below atmospheric pressure and to produce a superatmospheric pressure in the secondary tank about 2 pounds square inch greater than atmospheric pressure. A 2 pound per square inch pressure differential corresponds to a hy' drostatic head of approximately 4.3 feet. The stabilization system shown in FIG. 5 is double-acting in that it creates both subatmospheric and superatmospheric pressures in the secondary tank. This range of pressure differentials in the secondary tank corresponds to a total hydrostatic head differential of approximately 8.6 feet; that is. assuming a 2 pound per square inch pressure differential in secondary tank for both the suction and discharge connections of the blower to the secondary tank, A 11,, the distance of upper water level in the secondary tank above load waterline 96, can be 4.3 feet greater than the height of wave crest 113 above the load waterline. Similarly, A I1 the distance of the lower water level in the secondary tank below load waterline 96, can be 4.3 feet greater than the distance of the wave trough below the vessel load water' line as the vessel actually encounters the wave train.

As noted above, a pair of primary and secondary sta' bilization tanks, as shown in FIG. 5, is provided fore and aft in each of vertical hulls 12 and 14 of vessel 10. The four sets of stabilization tanks may be operated in phase with each other to counteract heave motions of the vessel if desired; in this case, mode selection device 111 is operated to its HEAVE selection state. On the other hand, by operation of the mode selection device to its PITCH state, the two pairs of forward tanks can be operated out of phase to the two aft pairs of tanks to counteract induced pitching motions of the vessel. Additionally, the starboard two pairs of tanks can be operated out of phase with the port two pairs of tanks to compensate for induced rolling motion of the vessel; in this case device 111 is operated to its ROLL state.

Preferably, operation of the mode selection device to its PITCH, HEAVE or ROLL status automatically selects the appropriate phase relationship between operation of the valves of the respective air flow means with respect to the time of arrival of the wave troughs and crests at the vessel. The precise time-phase relationship between the arrival of the wave crests and troughs at the vessel and the cycling of the valve of the several air flow means will be determined by the viscous damping characteristics of the hull in its various modes of motion and by the basic stability of the vessel as to these modes of motion. If desired, the mode selection device may include additional operative states for controlling operation of stabilization tanks at diagonally opposite quarters of the vessel in tandem with each other out of phase with the tanks at the other quarters of the vessel, as when the vessel encounters quartering. as op posed to beam or bow-on or following seas.

In order that control mechanism 110 may regulate the operation of valves 106-109 in the proper phase relation to induced motions of the vessel. or in the proper phase relation to the motion-inducing inputs applied to the vessel by passing waves. the mode selection device 111 receives signals 118 which are indicative either of actual induced motions in the vessel or of approaching motion-inducing inputs to the vessel. Signals indicative of actual induced motions may be generated by accelerometers (not shown) located in appropriate places in the vessel and connected appropriately to device 11]. Device 111 may select, depending on its operative state, between appropriate accelerometers, or it may analyze the several accelerometer output signals and, consistent with its operative state, transmit the correct information to control mechanisms 110. On the other hand, signals 118 indicative of approaching motioninducing inputs to the vessel, and also indicative of the timing and magnitude of such inputs, may be generated by a directionally adjustable wave scanner mounted to the vessel as shown in FIG. 1. If desired, signals 118, from whatever source, could be applied directly to control mechanism 110, the manner in which each control mechanism processes and applies such signals being governed by the operative state of mode selection device 111.

It will also be apparent that a stabilization system according to this invention can be used to advantage on crane barges and the like to compenstate for heel or trim of the barge, as when the crane if operating over the side or end of the barge, respectively. That is, a motion stabilization system according to this invention can be used to advantage on a crane barge to stabilize the vessel against undue motions induced by passing wave trains and also to maintain an even-keel attitude of the barge during static conditions.

Stabilization systems according to this invention may be used to advantage in semi-submersible drilling vessels. Semi-submersible drilling vessels are substantially transparent in terms of roll, pitch and heave to short period waves, but they are extremely sensitive in terms of heave to long period waves. Concentrically arranged primary and secondary stabilization tanks according to the foregoing description may be provided in the pylonlike cylindrical, surface-piercing portions of semisubmersible drilling rigs to produce significant heave stabilization effects on the rig when the rig encounters long period wave periods in resonance with the natural heave period of the rig.

The invention has been described above with reference to a presently preferred embodiment of the invention set in the context ofa presently preferred environment. Workers skilled in the art to which this invention pertains will readily appreciate that actively assisted pasive motion stabilization systems according to this invention may be used in vessels other than the particular configuration of vessel shown in the drawings. Such workers will also appreciate that modifications and variations in the structures and procedures described above may be made without departing from the basic teachings of this invention. Accordingly, the foregoing description should not be considered as limiting or defining all the various forms which this invention may take, either structurally or in terms of procedure.

What is claimed is:

l. A motion stabilization system for a floating vessel having a buoyant hull comprising a first tank extending vertically in the hull from a first selected location below the load waterline to a second selected location above the waterline and having at least one vertical boundary thereof defined by the hull shell, a plurality of vertically spaced openings from the tank through the shell, some of the openings being located below the waterline and some of the openings being located above the waterline, a second tank extending vertically in the hull from about the elevation of the first location to about the elevation of the second location, means providing water flow communication between the lower ends of the tanks and having an effective water flow rate therethrough greater than the effective water flow rate through said openings, and air flow means communi eating with the upper extent of the second tank operable alternately to generate superatmospheric air pres sure in the second tank and to generate subatmospheric air pressure in the second tank.

2. Apparatus according to claim 1 including means for operating the air flow means in a selected timed relation to movements of the load waterline relatively above and below the surface of water adjacent the exterior of the hull for generating said subatmospheric pressure in relation to said water surface being above the waterline and to generate said.superatmospheric pressure in relation to said water surface being below the waterline.

3. Apparatus according to claim 1 including control means for the air flow means comprising means for scanning waves and the like moving relatively toward the vessel and for operating the air flow means to counteract motions of the vessel induced by said waves encountering the hull.

4. Apparatus according to claim 3 wherein trol means is operable to operate the air flow counteract heave of the vessel.

5. Apparatus according to claim 3 wherein the vessel includes two pairs of said first and second tanks disposed along opposite sides of the hull and air flow means for the two second tanks, and the control means is operable to operate the air flow means to counteract roll of the vessel.

6. Apparatus according to claim 3 wherein the vessel includes two pairs of said first and second tanks disposed toward the opposite ends of the vessel and air fiow means for the two second tanks, and the control means is operable to operate the air fiow means to counteract pitch of the vessel.

7. Apparatus according to claim 1 wherein the air flow means comprises an air blower having a suction connection and a discharge connection, and duct means for separately coupling the suction connection to atmosphere and to the upper extent of the second tank and for separately coupling the discharge connection to atmosphere and to the upper extent of the second tank, and valve means for regulating each said coupling.

8. A method for stabilizing and counteracting the motions of a floating vessel induced by passing waves and the like comprising the steps of a. providing in the vessel alongside the hull shell a first tank which extends vertically from below the load waterline to above said waterline and which has a plurality of vertically spaced openings through the shell including above and below the waterline,

b. providing a second tank in the vessel adjacent the first tank, the second tank extending vertically above and below the waterline and communicating adjacent its lower end with the first tank adjacent its lower end via a duct which has an effective the conmeans to 1 l 12 water flow rate greater than the effective water d. generating sub-atmospheric pressure in the second flow rate by gmwty through Smd Openmgs tank during relative raising of water outside the c. generating super-atmospheric pressure in the second tank during relative lowering of water outside the hull below the load waterline thereby to subml the Second tank Wuh waterstantially empty the second tank of water. and

hull abm'e the load waterline thereby to suhstan 

1. A motion stabilization system for a floating vessel having a buoyant hull comprising a first tank extending vertically in the hull from a first selected location below the load waterline to a second selected location above the waterline and having at least one vertical boundary thereof defined by the hull shell, a plurality of vertically spaced openings from the tank through the shell, some of the openings being located below the waterline and some of the openings being located above the waterline, a second tank extending vertically in the hull from about the elevation of the first location to about the elevation of the second location, means providing water flow communication between the lower ends of the tanks and having an effective water flow rate therethrough greater than the effective water flow rate through said openings, and air flow means communicating with the upper extent of the second tank operable alternately to generate superatmospheric air pressure in the second tank and to generate subatmospheric air pressure in the second tank.
 1. A motion stabilization system for a floating vessel having a buoyant hull comprising a first tank extending vertically in the hull from a first selected location below the load waterline to a second selected location above the waterline and having at least one vertical boundary thereof defined by the hull shell, a plurality of vertically spaced openings from the tank through the shell, some of the openings being located below the waterline and some of the openings being located above the waterline, a second tank extending vertically in the hull from about the elevation of the first location to about the elevation of the second location, means providing water flow communication between the lower ends of the tanks and having an effective water flow rate therethrough greater than the effective water flow rate through said openings, and air flow means communicating with the upper extent of the second tank operable alternately to generate superatmospheric air pressure in the second tank and to generate subatmospheric air pressure in the second tank.
 2. Apparatus according to claim 1 including means for operating the air flow means in a selected timed relation to movements of the load waterline relatively above and below the surface of water adjacent the exterior of the hull for generating said subatmospheric pressure in relation to said water surface being above the waterline and to generate said superatmospheric pressure in relation to said water surface being below the waterline.
 3. Apparatus according to claim 1 including control means for the air flow means comprising means for scanning waves and the like moving relatively toward the vessel and for operating the air flow means to counteract motions of the vessel induced by said waves encountering the hull.
 4. Apparatus according to claim 3 wherein the control means is operable to operate the air flow means to counteract heave of the vessel.
 5. Apparatus according to claim 3 wherein the vessel includes two pairs of said first and second tanks disposed along opposite sides of the hull and air flow means for the two second tanks, and the control means is operable to operate the air flow means to counteract roll of the vessel.
 6. Apparatus according to claim 3 wherein the vessel includes two pairs of said first and second tanks disposed toward the opposite ends of the vessel and air flow means for the two second tanks, and the control means is operable to operate the air flow means to counteract pitch of the vessel.
 7. Apparatus according to claim 1 wherein the air flow means comprises an air blower having a suction connection and a discharge connection, and duct means for separately coupling the suction connection to atmosphere and to the upper extent of the second tank and for separately coupling the discharge connection to atmosphere and to the upper extent of the second tank, and valve means for regulating each said coupling. 